This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Flows including strong adverse pressure gradient and boundary layer separation from a smooth wall (as opposed to separation fixed at a geometric discontinuity) are among the most challenging high-Reynolds number flows to model. Accurate prediction of the separation point puts great demands on turbulence models, and small errors in separation location can produce large errors in the overall velocity and pressure distribution. The end result is that CFD has not yet made significant inroads into the design of systems like highly loaded pumps and air/hydrofoils operating at far-off-design conditions. The objective of this research is to provide a detailed data set for a three-dimensional, closed separation bubble formed on a smooth wall in a geometrically-simple diffuser geometry. In addition, the experimental program is coupled to a modeling effort. By doing the experiments and simulations in parallel, we can assure that the most detailed data are supplied in the flow regions causing the most difficulties for simulations.